Speaking in the wake of the attack on Charlie Hebdo in Paris, David Cameron announced a range of anti-terror laws. In the process, he implied that the government would be cracking down on anti-surveillance techniques.

“In extremis, it has been possible to read someone’s letter, to listen to someone’s call, to listen in on mobile communications,” he said. “The question remains: are we going to allow a means of communications where it simply is not possible to do that? My answer to that question is: no, we must not.”

What’s the problem?

There are a lot. For many, it’s simply a matter of principle: far from being unacceptable, the ability to have a conversation which the government can’t eavesdrop on is a crucial part of what it means to live in a democratic country.

But for others, the issue is the apparent hole in Cameron’s technical knowledge which led him to believe that what he was suggesting was even possible.

So what he asked for isn’t easy to do?

It’s practically impossible.

The government can eavesdrop on the post and phone calls because both of them rely on a trusted provider. You trust Royal Mail with access to all your letters, and in return it both keeps them safe from illegal eavesdropping, and hands it over to government authorities who make a valid legal demand for access.

Modern encryption doesn’t work like that. It forgoes any need for a trusted third-party to carry your messages, instead relying on complex maths to ensure that it is impossible for anyone other than the intended participant to read it.

There’s no way to give the government a way past those locks without also making it possible for any other attacker to walk in through the same back door – and that doesn’t just apply to communication technologies such as WhatsApp and iMessage, which are encrypted by default these days, but to everything from online banking to the very systems that Britain’s security services use to send state secrets back home.

How did Cameron’s suggestion go down?

Poorly. Security expert Graham Cluley said Cameron was in “cloud cuckoo land”, and tech start-ups said they would have to abandon the UK if his suggestions came to pass. One said that even the belief that it was a credible threat would “take the bottom out of internet businesses operating in the UK”.

Perhaps unsurprisingly, Downing Street scrambled to back-pedal the comments. Guy Levin, the head of tech lobbying group Coadec, said he’d been told by Number 10 “that [the Prime Minister’s] comments are not about banning encryption.” Instead, he was told the remarks were about application of two existing laws, the Data Retention and Investigatory Powers Act (Dripa) and the Regulation of Investigatory Powers Act (Ripa), “and finding a way to work with [internet companies] to deliver on them.”

Is that the end of it, then?

Not by a long shot. While Cameron’s first attempt to use the Charlie Hebdo attack to crack down on tech may have been solidly rebuffed, the Prime Minister has an ally in the form of President Obama, himself smarting over the fallout of the Sony Pictures hack.

The pair are meeting in Washington, and are expected to announce a combined security effort. But at the same time, Cameron won’t pass up the opportunity to lobby the American President for stronger tools, both legal and technical, to counter the normalisation of encryption in everyday life.